1. Field of Invention
The present invention pertains to an image capture panel for capturing direct radiographic images. More particularly, the present invention pertains to a new and novel method and apparatus for removing the trapped counter charges at the interface between an imager dielectric layer and the photoconducting layer.
2. Description of Related Art
Digital X-ray radiogram can be produced by using layers of radiation sensitive materials to capture incident X-ray as image-wise modulated patterns of light intensity (photons) or as electrical charges. Depending on the intensity of the incident X-ray radiation, electrical charges generated either electrically or optically by the X-ray radiation within a pixel area are quantized using a regularly arranged array of discrete solid state radiation sensors. U.S. Pat. No. 5,319,206 (Lee, et al.) and assigned to E. I. du Pont de Nemours and Company, describes a system employing a layer of photoconductive material to create an image-wise modulated areal distribution of electron-hole pairs which are subsequently converted to corresponding analog pixel (picture element) values by electro-sensitive devices, such as thin-film transistors. U.S. Pat. No. 5,262,649 (Antonuk et al.) describes a system employing a layer of phosphor or scintillation material to create an image-wise modulated distribution of photons which are subsequently converted to a corresponding image-wise modulated distribution of electrical charges by photosensitive devices, such as two dimensional amorphous silicon photodiodes. These solid state systems have the advantage of being useful for repeated exposures to X-ray radiation without consumption and chemical processing of silver halide films.
In Indirect Conversion systems (e.g. U.S. Pat. No. 5,262,649) that utilize a scintillation material to create an image-wise modulated distribution of photons from the absorbed X-ray energy, photons generated from the absorbed X-ray may undergo multiple scattering or spreading before they are detected by the two dimensional photosensitive device, resulting with degradation of image sharpness or a lower MTF (Modulation Transfer Function). The degradation of image sharpness is significant especially for thicker layer of scintillation material is required to capture sufficient x-ray quanta for image forming.
In Direct Conversion systems (FIG. 1) utilizing a photoconductive material, such as selenium described in U.S. Pat. No. 5,319,206, before exposure to image-wise modulated X-ray radiation, an electrical potential is applied to the top electrode 100 to provide an appropriate electric field. During exposure to X-ray radiation 111, electron-hole pairs (indicated as − and +) are generated in the photoconductive layer 104 (referred to in FIG. 1 as “X-ray Semiconductor”) in response to the intensity of the image-wise modulated pattern of X-ray radiation, and these electron-hole pairs are separated by the applied biasing electric field supplied by a high voltage power supply 120 (e.g., programmable). The electron-hole pairs move in opposite directions along the electric field lines toward opposing surfaces of the photoconductive layer 104 (also referred to as an X-ray semiconductor). After the X-ray radiation exposure, a charge image is stored in the storage capacitor 112 of the TFT array 112A. This image charge is then readout by an orthogonal array of thin film transistors 110 and charge integrating amplifiers 118. In Direct Conversion systems, since the electric field is perpendicular to the charge collection electrode 106, the image sharpness or MTF is preserved regardless of the thickness of the photoconductive material. Thicker X-ray conversion material can be used to absorb sufficient X-ray energy without compromising the resulted image quality. As also shown in FIG. 1, the device includes a dielectric layer 102, an electron blocking layer 108 and a glass substrate 122.
In Direct Conversion systems, the bias voltage is applied between an upper electrode 100 and the ground plane connected to the charge collection electrodes 106. In one design, this upper electrode 100 can be insulated from the photoconductive material 104 by a thin layer of dielectric material 102, such as the system described in U.S. Pat. No. 5,319,206, no charge can be injected from the high voltage bias electrode to the selenium layer. In another design, the upper electrode can also be connected to the photoconductive material via a non-insulated charge blocking layer such as U.S. Pat. No. 7,233,005 (Bogdanovich, et al.) or via a highly doped N-type or P-type semiconducting non-insulated layer structure, such as U.S. Pat. No. 6,171,643 (Polischuk, et al.), to serve as a charge blocking layer. For systems using non-insulating layer, a certain amount of leakage current is always present. Furthermore, after X-ray exposure, charges accumulated at the upper interface eventually need to pass through or break through this layer and return to the power supply. The current that passes through or break through this charge blocking layer will gradually weaken the charge blocking and the leakage current will increase in time especially in areas of high radiation exposure. With the design where the upper electrode 100 is totally insulated from the photoconductive layer 104 by dielectric material 102, no current is allowed to pass through or break through and therefore the charge injection blocking can be maintained. However, the accumulated charges in the dielectric-photoconductive layer need to be removed or returned to ground after the X-ray exposure. Methods to remove these trapped charges have been described such as that described in U.S. Pat. No. 5,563,421 (Lee, et al.).
All references cited herein are incorporated herein by reference in their entireties.